Atomistic Insight into the Epitaxial Growth Mechanism of Single-Crystal Two-Dimensional Transition-Metal Dichalcogenides on Au(111) Substrate

A mechanistic understanding of interactions be-tween atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) and their growth substrates is important for achieving the unidirectional alignment of nuclei and seamless stitching of 2D TMD domains and thus 2D wafers. In this work, we conduct a cross-sectional scanning transmission electron microscopy (STEM) study to investigate the atomic-scale nucleation and early stage growth behaviors of chemical vapor deposited monolayer (ML-) MoS2 and molecular beam epitaxy ML-MoSe2 on a Au(111) substrate. Statistical analysis reveals the majority of as-grown domains, i.e., similar to 88% for MoS2 and 90% for MoSe2, nucleate on surface terraces, with the rest (i.e., similar to 12% for MoS2 and 10% for MoSe2) on surface steps. Moreover, within the latter case, step-associated nucleation, similar to 64% of them are terminated with a Mo-zigzag edge in connection with the Au surface steps, with the rest (similar to 36%) being S-zigzag edges. In conjunction with ab initio density functional theory calculations, the results confirm that van der Waals epitaxy, rather than the surface step guided epitaxy, plays deterministic roles for the realization of unidirectional ML-MoS2 (MoSe2) domains on a Au(111) substrate. In contrast, surface steps, particularly their step height, are mainly responsible for the integrity and thickness of MoS2/MoSe2 films. In detail, it is found that the lateral growth of monolayer thick MoS2/MoSe2 domains only proceeds across mono-Au-atom high surface steps (similar to 2.4 angstrom), but fail for higher ones (bi-Au atom step and higher) during the growth. Our cross-sectional STEM study also confirms the existence of considerable compressive residual strain that reaches similar to 3.0% for ML-MoS2/MoSe2 domains on Au(111). The present study aims to understand the growth mechanism of 2D TMD wafers.